1. Technical Field
Aspects of the invention relate to a communication device (or a device having a communication function) that allows a voice communication to be performed with another communication device connected with a communication circuit network.
2. Related Art
Communication devices (e.g., a telephone) which allows a voice communication are well known. Such a communication device is typically configured as follows to realize the voice communication.
First, a communication device (calling station terminal) connected to a communication circuit network (e.g., a telephone circuit network) calls another communication device. Then, an exchanger of the communication circuit network transmits a CI (Call Indicator) signal for notifying a call to a called station terminal. The called station terminal outputs a ringing tone notifying a user of reception of a call in response to reception of the CI signal.
Thereafter, when the user of the called station terminal carries out an operation to start the communication, i.e., when the user picks up a hand set thereof as the ringing tone is output, the notification of the call with the CI signal from the communication circuit network is stopped, communication lines for the called station terminal and the calling station terminal are connected, thereby enabling a voice communication therebetween.
Specifically, in the called station terminal, when the operation to start the communication is carried out, a connection of an inner circuit to be connected to the communication circuit network is switched from a circuit that detects a reception of the CI signal to a circuit connected to the handset (i.e., a circuit used for transmitting/receiving voice signals). For example, with use of a relay (off-hook relay), a circuit including the handset must be closed. An example of such a configuration is disclosed in Japanese Patent Publication No. 3396225.
The notification using the CI signal as described above is stopped after the exchanger of the telephone circuit network has detected that the operation for starting the communication has been done at the called station terminal. Therefore, the CI signal may flow through the circuit including the handset when such an operation for starting the communication.
The CI signal is typically defined as a signal having an ON state and an OFF state which sequentially occur. When in the ON state, the CI signal has repeatedly alternating H (high) level and L (low) level, and in the OFF state, the CI signal has a continuous L level. If the communication start operation is carried out when the CI signal is in the OFF state, the pulses (i.e., alternating H and L levels) will not flow through the circuit. If the CI signal is in the ON state when the communication start operation is carried out, although within a very short period, the pulses constituting the CI signal flow through the circuit connected to the handset. In such a case, non-voice signal is output through the handset as an audible sound, which may provide an unpleasant noise to the user of the called station terminal.
Further, the amplitude of the pluses constituting the CI signal is generally greater than that of the voice signal exchanged during the voice communication. If the pluses are transmitted to the circuit to the handset, electronic elements included in the circuit may be damaged or broken as the signal level exceeds the durable voltages of the elements. Therefore, in view of protection of the circuit, it is also necessary to prevent the CI signal from flowing to the circuit.